1. Field of the Invention
The present invention refers to nonlinear optics (NLO) polymers and optical waveguides based thereon.
2. Description of the Related Art
The deployment and growth in performance of optic communication systems take advantage of the possibility of directly switching and processing optical signals without recurring to conversion into electronic format followed by retransmission. In dependence on the physical mechanism adopted, the so called “all-optical switching” can provide speed of response, transparency to modulation formats and also simultaneous processing of multiple wavelengths, as in the case of wavelength-division multiplexed (WDM) signals. At the basis of all-optical processing of signals stands the possibility of affect at least one among the propagation parameters of an optical beam by means of a second optical beam. This means that either amplitude, or phase, or state of polarization of the beam to be processed are affected by a second light beam interacting with the first. It is well known that optical beams interact with each other within a material through optical nonlinear effects. In particular, the third order dielectric susceptibility of a material, represented by the coefficient χ(3)(−ω4;ω1,ω2,ω3) is at the origin of third-order optical nonlinearities. In particular, when a pair of interacting optical beams are considered, a nonlinear variation of the refractive index in the material can be induced as proportional to Re χ(3)(−ωs;ωp,−ωp,ωs), where the real part of the nonlinear susceptibility is considered and subscripts s and p indicate respectively the interacting signal and pump beams. This effect is named ‘non-degenerate optical Kerr effect’, and the corresponding nonlinearly induced dephasing is named ‘cross-phase modulation (XPM)’ and is such that             Δ      ⁢                           ⁢              ϕ        s              =                                        2            ⁢                                                   ⁢            π                                λ            s                          ⁢        L        ⁢                                   ⁢        Δ        ⁢                                   ⁢        n            =                                    2            ⁢                                                   ⁢            π                                λ            s                          ⁢        L        ⁢                                   ⁢                  n          2                ⁢                  I          p                      ,where λs is the optical wavelength of the signal beam in vacuum, L is the effective interaction length,       n    2    =            3      ⁢      Re      ⁢                           ⁢                        χ                      (            3            )                          ⁡                  (                                                    -                                  ω                  s                                            ;                              ω                p                                      ,                                          -                                  ω                                      p                    ,                                                              ⁢                              ω                s                                              )                            4      ⁢                           ⁢              ɛ        0            ⁢              cn        2            is the Kerr coefficient expressed in m2W−1, and Ip is the intensity of the pump beam.
The Kerr-induced dephasing can be exploited to perform all-optical switching or processing of signals in an interferometric arrangement. This process is well established in literature, see for instance Kerr-induced switching in non-linear fiber loop mirrors (NLOM). One typical interferometric structure, used in optic communications is the Mach-Zehnder interferometer. In it a laser beam propagates in optical waveguides that are essentially channels of dielectric material surrounded by a cladding or substrate material having a lower index of refraction. The light beam originally propagates into one waveguide, which eventually splits into two dielectric paths, called ‘arms’. The optical power is therefore divided between the arms and recombines at the end of them. If the beams propagating in the two arms undergo the same phase shift, corresponding to the ‘balanced’ case, constructive interference occurs in recombination and full optical power is transmitted. The existence of a dephasing between the two arms causes a transmission loss. If the dephasing amounts to π radians, destructive interference occurs and no optical power is transmitted. The Mach-Zehnder interferometer is at the basis of integrated-optics electro-optical modulators. In this case, the waveguiding structure is realized in an electro-optical crystal and the dephasing between the arms is induced through the linear electro-optic effect, by suitably applying an electric voltage that causes a corresponding change in the refractive index.
The Mach-Zehnder structure can also be exploited all-optically, through the Kerr-induced XPM. In this case, an intensity modulated pump beam is forced into one of the arms of the interferometer, so that the refractive index in the same arm is accordingly modified and a phase unbalance is generated between the two arms. A suitable intensity modulation of pump beam is translated into phase modulation of the portion of signal beam in the activated arm and therefore into modulation of the unbalance and the transmitted signal intensity. If the unbalance of the interferometer is switched between zero and π radians, an ON/OFF switching of the signal beam can be performed. An example of Kerr nonlinearity used for switching a Mach-Zehnder interferometer between ON and OFF transmission states is given in (EP97119344). In the application, the ON-OFF switching is used to impress intensity modulation on the signal. One major problem of the cited reference is that the application relies on the extremely low n2 value of silica-based optical fiber (χ(3)=2.8×10−14 esu leading to n2=2.3×10−20 cm2W−1), so that a π dephasing can be obtained at the expense of kilometric interaction length and this constitutes an obstacle to integration of the device in more complex processing structures.
In order to carry out relatively compact structures performing all-optical processing of signals through third-order nonlinearity, it is necessary to adopt optical materials which conjugate high χ(3) values (at least χ(3)=10−11 esu) with low absorption at the wavelengths of interest. In this way, interaction lengths in the cm or tens of cm range become sufficient. Moreover, the material to be used must be technologically processable, so that it enables to design and implement optical waveguides.
It is known, for instance from U.S. Pat. No. 5,323,482 and Tarabia et al. J. Appl. Phys. 83(2)1998, that organic compounds which possess a noncentrosymmetrical structure, in fact defined as nonlinear, can be used in nonlinear optics (NLO).
For instance (H. S. Nalwa, Handbook of Organic Conductive Molecules and Polymers, Vol. 4, 1997), π-conjugated polymers are suitable as they have a dual role by exhibiting large third-order χ3 coefficient and electrical conductivity values. It is therein reported that third-order NLO effects have been investigated in a variety of organic molecular and polymeric systems such as liquids, dyes, charge transfer complexes, π-conjugated polymers, NLO dye-grafted polymers, organometallic compounds, composites and liquid crystals. The importance of organic polymers has been realized with reference to large nonlinear optical properties, high optical damage thresholds, ultrafast optical responses, architectural flexibility and ease of fabrication. Third-order optical nonlinear values determined by for example THG, DFWM, and self-focusing techniques greatly differ from each other due to the distinct nonlinear optical processes and because of the applied experimental conditions such as the measurement wavelength and environmental conditions. Third-order optical nonlinearity values are often quoted as resonant and nonresonant values resulting from their wavelength dispersion within or far from the optical absorption regions of non linear materials. The resonant χ3 values can be several orders of magnitudes larger than that of the non resonant value.
For instance the aforementioned Tarabia et al. J.Appl.Phys. 83(2)1998 shows in particular that deuterated poly(phenylenevinylene) (D-PPV) are useful conjugated polymers for preparing self-assembled heterostructures with alternate different semiconducting layers having different dielectric constants.
However, one of the major problems to be solved regards the possibility of simultaneously reaching high χ(3) values and low optical absorption. Since the interaction length, in the absence of dispersion, is ruled by absorption and an effective length is defined as L=α−1 where α is the linear absorption coefficient of the material.